Protein kinases (PKs) are enzymes that catalyze the phosphorylation of hydroxy groups on tyrosine, serine and threonine residues of proteins. Receptor tyrosine kinases (RTKs) are a sub-family of protein kinases that play a critical role in cell signaling and are involved in the process of tumorigenesis including cell proliferation, survival, angiogenesis, invasion and metastasis. A class of RTK known as the type III receptor tyrosine kinase family, which includes the receptors PDGFR α, PDGFR β, FLT3, KIT, VEGFR and CSF1R, has been implicated in various proliferative and inflammatory diseases.
CSF1R (also known as macrophage colony stimulating factor receptor (M-CSFR) or fms) is a receptor for the macrophage colony stimulating factor (M-CSF or CSF-1). Binding of the CSF-1 ligand to its receptor results in dimerization and auto-phosphorylation of the receptor and leads to activation of downstream signal transduction pathways including the PI3K/Akt and the mitogen activating protein kinase MAPK pathways. Activation of CSF1R leads to the proliferation, survival, motility and differentiation of cells of the monocyte/macrophage lineage and hence plays a role in normal tissue development and immune defense. Activation of CSF1R also leads to the proliferation and differentiation of osteoclast precursors and therefore mediates the process of bone resorption.
Because of its role in osteoclast biology, CSF1R is believed to be an important therapeutic target for osteoporosis and inflammatory arthritis. For example, elevated M-CFS signaling leads to elevated osteoclast activity, which leads to bone loss attending arthritis and other inflammatory bone erosion. (See Scott et al. Rheumatology 2000, 39: 122-132, Ritchlin et al. J. Clin. Invest. 2003, 111:821-831). Inhibition of CSF1R therefore represents a promising therapeutic approach for arthritis and other inflammatory bone erosion which is further supported by the efficacy data of known CSF1R inhibitors such as Ki-20227 and GW2580 in arthritic animal models (See Conwat et al. JPET 2008, 326:41-50 and Ohno et al. Eur. J. Immunol. 2008, 38:283-291). Dysregulation of osteoclast development and disruption in the balance of bone resorption and bone formation that underlie osteoporosis might also be treated with a modulator of CSF1R.
Elevated expression or activation of CSF1R and/or its ligand have been found in patients with acute myeloid leukemia, prostate, breast, ovarian, endometrial, colorectal, pancreatic and a variety of other cancers, and elevated levels of M-CSF is associated with poor prognosis in certain cancers (See, Muller-Tidow et al. Clin Cancer Res, 2004, 10:1241-1249, Bauknecht et al. Cancer Detect. Prev., 1994, 18: 231-239; Baiocchi G et al. Cancer 1991, 67:990-996; Kirma et al Cancer Res. 2007; Sapi et al. Exp. Biol. Med., 2004, 229:1-11; Kluger et al. Clin. Canc. Res. 2004 10:173-177; Mroczko et al., Clin. Chem. Lab. Med. 2005 43:146-50 and Mroczko et al., Clin. Chim. Acta 2007, 380:208-212). The data suggests that CSF1R may be a valuable therapeutic target for these solid tumors.
Early studies have associated elevated expression of M-CSF with increased leukocyte infiltration of solid tumors in human breast and ovarian cancers (Scholl et al. J. Natl. Cancer Inst. 1994, 86:120-126, Tang et al. J. Cell. Biochem. 1990, 44:189-198). Further studies have shown that M-CSF is one of several cytokines implicated in the recruitment of tumor-associated macrophages (TAMs) that contribute to tumor angiogenesis and tumor progression to metastasis, and more recently, that the preclinical inhibitor GW2580 inhibits tumor metastasis and angiogenesis in mice tumor xenograft experiments (Priceman et al. Blood 2010 115(7):1461-1471). Stimulated osteoclast activity is also believed to underlie the pathophysiology of bone metastases. (Lipton, J. Support. Oncol. 2004 2:205-220). Metastatic bone lesions results in significant localized bone loss and lead to skeletal morbidity, symptoms which include bone pain, bone fractures and hypercalcemia. Inhibition of CSF1R therefore may therefore provide therapy for solid tumors and metastatic cancer including metastases to the bone.
Another member of the PDGFR family, FLT3 (also called Flk2), plays an important role in the proliferation and differentiation of hematopoietic stem cells and activating mutation or overexpression of this receptor is found in AML (See, Heinrich Mini-Reviews in Medicinal Chemistry 2004, 4(3):255-271, Kiyoi et al. Int J Hematol, 2005 82:85-92). More than a dozen known FLT3 inhibitors are being developed and some have shown promising clinical effects against AML (See Levis et al. Int J Hematol. 2005 82:100-107). The FLT3 receptor is also expressed in a large portion of dendritic cell progenitors and stimulation of the receptor causes the proliferation and differentiation of these progenitors into dendritic cells (DC). Since dendritic cells are the main initiators of the T-cell mediated immune response, including the autoreactive immune response, FLT3 inhibition is a mechanism for downregulating DC-mediated inflammatory and autoimmune responses. One study shows the FLT3 inhibitor CEP-701 to be effective in reducing myelin loss in experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis (See Whartenby et al. PNAS 2005 102: 16741-16746). A high level of the FLT3 ligand is found in the serum of patients with Langerhans cell histiocytosis and systemic lupus erythematosus, which further implicates FLT3 signaling in the dysregulation of dendritic cell progenitors in those autoimmune diseases (See Rolland et al. J. Immunol. 2005 174:3067-3071).
KIT (or stem cell factor receptor, or SCFR) is another member of the RTK family, and the presence of kit mutations is a key diagnostic marker for gastrointestinal stromal tumors (GIST) (Duensing et al. Cancer Investigation 2004, 22(1): 106-116). Gleevec® (imatinib mesylate or STI571), the first FDA-approved RTK inhibitor originally approved for c-Abl-mediated chronic myeloid leukemia, gained FDA-approval for KIT-mediated GIST in 2002 and has validated the molecular-based approach of Kit inhibition for the treatment of GIST. (Giorgi and Verweij, Mol. Cancer Ther. 2005 4(3):495-501). Gain of function mutations of the Kit receptor are also associated with mast cell/myeloid leukemia and seminomas/dysgerminomas (Blume-Jensen Nature 2001 411(17): 355-365. KIT mutations have been also identified in certain melanomas and is recognized as a potential therapeutic target for melanoma (Curtain et al. J Clin. Oncol. 2006 24(26):4340-4346).
There continues to be a need for the identification of small molecules that inhibit RTKs, particularly compounds useful for the treatment of CSF1R-, FLT3, PDGFRβ- and/or KIT-mediated diseases.